1. Add onion and garlic to pan on medium high heat with some oil. Once the onion becomes translucent, add the celery and cook until softened. Add the mushroom and kale, and cook until the kale has wilted and the mushrooms have released their liquid.
2. Preheat oven to 375 degrees F. In a large casserole dish, combine all of the veggies and season. Mix the tomato puree and leftover gravy, and and pour enough to cover the veggies.
3. Spoon your potatoes over the top, as thick as you would like. Bake for 45 minutes-1 hour, or until veggies are tender and potatoes are browned.